The invention relates to a circuit for the generation of a control variable for the control of the transfer constant of electronically actuable amplifying or attenuating members.
In systems for the automatic dynamics compression and expansion it is essential to have an exactly reproducible relation between the transfer constant of the controllable amplifying and attenuating members and of the relevent useful signal input amplitude of the compressor or expander. This clear relationship between the said variables is an important prerequisite for the essential feature that the frequency spectrum and the level conditions of the reproduced useful signal correspond to those of the original. Beyond this there is another basic requirement for the expander input level to be absolutely identical to the compressor output level.
This second requirement is not always fulfillable without adjustment or compensation. It is for instance possible for level faults to occur in magnetic tape recording apparatus which are due to different types of tape or when the tapes are run off on several sets which have been differently adjusted.
The aforesaid second requirements can be dispensed with under the following conditions: level errors, also designated by the term "mistracking", do not exercise any influence on the level conditions of the useful signal when the characteristic curve for the compandor is "dB-linear" i.e. when it is linear (when plotted) against the logarithm of the input amplitude.
A transfer element with preferably exponentional characteristics is inserted between a charging capacitor in the control voltage generator and the control inputs of the electronically controllable amplifying or attenuating members for the achievement of the desired control characteristic (curve). This transfer element may consist of a diode or a transistor in which the exponential relationship between conducting state current and forward voltage of the semiconductor film is utilised. Only a few specific types of these exhibit good linearity over several decades.